Guide Updated Oct 6, 2026

What Are Critical Lab Values?

A critical lab value, also called a panic value, is a result so far outside the usual range that it may signal a life-threatening problem. The lab must report it to your doctor right away. Each lab sets its own limits: Labcorp alerts on potassium below 2.5 mmol/L, ARUP below 3.0.

Seeing a critical result right now? If you feel unwell, don't wait for a callback: get urgent care or call emergency services. If you feel fine, call your doctor's office today, or its after-hours line if it's closed.

The short version

  • A critical (panic) value is past a second limit, much wider than the reference range. It's set there because it may signal immediate danger.
  • Federal lab rules require the lab to alert whoever ordered the test right away. The call goes to your doctor's office, not to you.
  • Each lab sets its own limits. For potassium, Labcorp alerts below 2.5 or above 6.5 mmol/L, while ARUP alerts below 3.0 or above 6.1.
  • Your portal shows a critical result and your doctor hasn't called? Call the office the same day. If you feel unwell, get urgent care.

Most flags on a lab report mean "outside the usual range". A critical value means something else. Your number is far enough out that the lab treats it as a possible emergency and picks up the phone.

That difference matters when you're reading results alone in a patient portal, often before your doctor has seen them.

What is a critical lab value?

A critical value, also called a panic or alert value, crosses a limit the lab sets for possibly life-threatening problems. Labcorp, for example, calls critical results life-threatening. Its rules say the doctor or care team must be told immediately.

Critical is not the same as abnormal. Labcorp's list says so directly: a result outside the normal range may be abnormal, but the two words aren't interchangeable.

2.5 3.5 5.2 6.5
Critical
L
Reference range
H
Critical
Potassium for adults at Labcorp, in mmol/L. Most out-of-range results land in the H or L zones. Only the outer zones count as critical.
Abnormal result Critical result
How far out Outside the reference range Past a much wider alert limit
Report code H, L or A HH, LL, AA, or a "critical" label
Who is told It appears on your report and in the portal The lab must alert the ordering clinician immediately
What to do Discuss it with your doctor Contact your doctor the same day, or get urgent care if unwell

Critical lab values list: Labcorp vs ARUP

Labs keep critical limits for a short list of tests where a value can turn dangerous fast: electrolytes, blood sugar, calcium, the main blood counts and clotting. Here's how two large US labs set them for adults.

Blood chemistry Labcorp ARUP
Potassium Below 2.5 or above 6.5 mmol/L Below 3.0 or above 6.1 mmol/L
Sodium Below 120 or above 160 mmol/L Below 120 or above 160 mmol/L
Glucose Below 40 or above 500 mg/dL Below 55 or above 450 mg/dL
Calcium, total Below 7.0 or above 13.0 mg/dL Below 6.0 or above 13.0 mg/dL
Calcium, ionized Not on its list 0.78 or less, or 1.58 or more mmol/L
Magnesium Not on its list Below 1.0 or above 9.0 mg/dL
Phosphorus Not on its list Below 1.0 or above 9.0 mg/dL
Bicarbonate (total CO2) Not on its list Below 10 or above 40 mmol/L
Bilirubin, total Above 17.0 mg/dL Above 15 mg/dL
Lactate Not on its list 5 mmol/L or more*
Troponin I, high-sensitivity Not on its list 200 ng/L or more*
Blood counts and clotting Labcorp ARUP
Hemoglobin Below 6.1 or above 21.4 g/dL Below 7.0 g/dL*
Hematocrit Below 18.1% or above 64.4% Below 21%*
Platelets Below 21 or above 999 ×10³/µL 20 or less, or 1,000 or more ×10³/µL*
White blood cells Below 1.1 or above 49.9 ×10³/µL 2.0 or less, or 40 or more ×10³/µL*
INR Above 4.9 5.0 or higher
aPTT (PTT) Above 89 seconds 85 seconds or more
Fibrinogen Below 80 or above 999 mg/dL Below 100 mg/dL

*ARUP runs these tests only for University of Utah Health. Labcorp uses the same call limits at every age. ARUP sets separate limits for newborns on a few tests, including glucose and hemoglobin; these tables show adult limits. Both lists also include results that aren't numbers, such as certain dangerous infections found in a culture.

The two lists agree on which tests matter, but not on exactly where the lines sit. So treat any published list, this one included, as an example. The limits of the lab that ran your test are the ones that apply to you.

What happens when a lab finds a critical value?

The lab confirms the number, then phones whoever ordered the test. Labcorp, for instance, reports a critical result only after it's verified and entered into its system.

Federal lab rules require that call to happen right away. Labcorp's policy names your doctor, their representative or other staff caring for you. The call goes to the clinic, not to you.

Your doctor's office decides what happens next. You might get a call, a repeat test, a change in medicine or a referral to the emergency room.

Can a critical value be wrong?

Sometimes the sample, not your body, is the problem. Clenching and relaxing your fist during a blood draw can briefly raise potassium, MedlinePlus notes. That can lead to a wrong result.

That's one reason labs double-check critical results, and your doctor may order a repeat test. But it's never a reason to ignore one. Only a doctor can tell a sample problem from a real emergency.

What to do if you see a critical result

If you feel unwell, don't wait for a callback. Chest pain, trouble breathing, a racing or irregular heartbeat, confusion, fainting or severe weakness all need urgent care or emergency services now.

If you feel fine:

  • Call your doctor's office the same day, or its after-hours line if it's closed. Say the portal shows a critical result, and ask if they got the lab's call.
  • Have the report open with the test name, your value and the flag.
  • Don't try to correct the number yourself with food, fluids or supplements before you've spoken to a clinician.
  • Ask whether the test should be repeated.

Questions to ask about a critical result

  • Did you receive the call from the lab about this result?
  • Could anything about the sample or the draw explain it?
  • Do I need a repeat test, and how soon?
  • Is there anything I should or shouldn't do until then?
After you've spoken to your doctor

Keep the repeat test next to the first one.

When the follow-up result comes in, upload both reports. They sit side by side on one timeline, each with the range its lab printed, so you can see what changed.

Works with Quest, Labcorp, MyChart & PDFs

Potassium mmol/L

First result · Mar 4

6.8

Lab range 3.5–5.2

Repeat test · Mar 5

4.6

Lab range 3.5–5.2

Both results on one timeline

Critical value questions

What is a critical lab value?
It's a result that crosses a limit the lab sets for possibly life-threatening problems. Labs must report it to your doctor right away, without waiting for the routine report.
Are panic values and critical values the same thing?
Yes, for the most part. Panic value and critical value name the same idea; Labcorp's list, for example, is titled "Critical (Panic) Values". Some labs say alert value instead. On a report they often appear as HH, LL or AA, or are labeled critical.
What is the difference between an abnormal and a critical lab value?
An abnormal result is outside the reference range. That's common, even in healthy people. A critical result is past a much wider alert limit that may signal an emergency. The lab must report it to your doctor right away.
Will the lab call me if my result is critical?
Usually not directly. The lab must alert whoever ordered the test, typically your doctor's office. The office then decides how to contact you. If your portal shows a critical result and nobody has called, phone the office the same day.
What potassium level is critical?
It depends on the lab. Labcorp alerts below 2.5 or above 6.5 mmol/L, while ARUP alerts below 3.0 or above 6.1 mmol/L. The limits used by the lab that ran your test are the ones that count.
What blood sugar level is critical?
For adults, Labcorp alerts on glucose below 40 or above 500 mg/dL, and ARUP below 55 or above 450 mg/dL. Your lab's own list and the flag on your report are what apply to you.

Sources

Written by BloodSight Editorial Team · Last updated

Disclaimer

This content is for informational and educational purposes only. It is not intended as medical advice, diagnosis, or treatment recommendation. Reference ranges may vary by laboratory. Always discuss your results with a qualified healthcare professional.

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